1. Field of the Invention
The present invention relates to AM radio broadcast receivers or AM (Amplitude Modulation) tuners. IN in particular, the invention relates to a method and to a relative circuit for preventing the generation of high frequency harmonics (tweet) when the receiver is out of tuning, i.e. when searching for broadcasting stations.
2. Description of the Prior Art
In AM receivers of a certain quality, a single or a double conversion superheterodyne circuit is invariably used.
In these receivers also invariably present is an automatic gain control (AGC) circuit, whose primary function is to keep the signal output of the amplifier (receiver) at a substantially constant level, i.e. a constant audio output for changing antenna's signal levels.
Such a gain control is automatically actuated, in the sense that when the receiver is tuned to a strong signal broadcasting station, the modulation signal received is instrumental in decreasing the gain of the receiver by decreasing the gain of the amplifying stages; conversely the gain is maximum when the signal received is very weak.
The automatic gain control permits maintaining a constant audio intensity reproduction notwithstanting the fact that the incoming electromagnetic signal has a continuously varying intensity because of propagation irregularities, especially in the case of distant broadcasting stations. In the absence of such an automatic gain control, the reception would be subject to continuous fluctuations (fading) and for this reason the automatic gain control is also often called antifading control. Another advantage of the AGC is the reduction of local disturbances when listening to nearby or powerful broadcasting stations.
The AGC is realized by detecting the modulated signal in the intermediate frequency (IF) stage of the receiver and extracting from the modulation envelope, by means of a suitable filter, the dc component of the modulating signal. The dc component is sensibly proportional to the amplitude of the carrier signal and therefore may be utilized for automatically adjusting the gain of the amplifying stages of the IF stage and, preferably, also the gain of the amplifying stages of the radio frequency (RF) stage. Therefore, when the receiver is tuned, the dc component of the detected signal is applied as an adjustment bias voltage or current to the various amplifying stages for reducing the gain and for keeping compressed the modulated signal going through the tuner stages so as to keep an appropriate dynamic range throughout the receiver and in particular in the mixer stage.
Obviously such an AGC system is incapable of intervening when the receiver is out of tuning, for example during the search for broadcasting stations. In practice, the AGC functions only when the amplitude of the received signal is above a certain minimum value (threshold value) so as to leave available the full sensitivity (gain) of the receiver when the incoming signal is very weak.
The dc component of the detected signal of the AGC system is also often used for driving a tuning indicator, i.e. a visual display of the actual tuning degree.
On the other hand, it is very desirable that the mixer of the receiver (i.e. the signal converting stage from a high frequency (RF) signal to an intermediate frequency (IF) signal) be kept in a proper dynamic working range in all conditions, especially when the receiver is out of tuning in order to optimize the performances of the system in terms of intermodulation and cross modulation.
According to the present art, often used is a second, distinct, automatic gain control system, commonly known as wide band RF-AGC, in the radio frequency stage of the receiver.
For distinguishing the two separate automatic gain control systems, the traditional AGC system already described is often referred to as narrow band AGC.
Different from the narrow band AGC, the intervention of the RF-AGC system consists in attenuating excessively strong signals which exceed the dynamic range of the RF stage, without discriminating between in-tune and out-of-tune signals. Such a type of intervention has undoubtedly disadvantageous aspects.